Holography is well known and widely used in many commercial applications including display holography, security, advertising and holographic optical elements and gratings. A holographic image is produced when light is diffracted at a complex spatially varying diffraction grating, which, in the simplest terms, re-directs the light towards the viewer in such a way as to give the illusion that the light is coming from a solid three-dimensional object.
This diffraction grating is produced by exposing a suitable photosensitive material to the optical interference pattern produced when two coherent light beams (usually produced by a laser) meet. The material records the variation in light intensity (as a variation in refractive index, absorption or thickness) and a corresponding diffraction grating results. If both light beams are simple collimated beams, the result will be a simple diffraction grating whose spatial period depends on the angle between the recording beams.
If the diffraction grating is illuminated with one of the recording beams (or a similar beam) it will diffract the light to reproduce the other recording beam. If one of the beams is a complex wavefront coming from a three dimensional object, the recorded diffraction grating will have the property that it can reconstruct this wavefront when illuminated with the other beam.
A wide variety of photosensitive materials are available including photopolymers, silver halides, dichromated gelatin, photo resists, thermoplastics, photochromics and photorefractive materials.
The recording process in photopolymers is described in detail in later sections.
Silver halide emulsions are widely used as holographic recording materials because of their sensitivity and their commercial availability. In addition these materials can be dye sensitized so that their spectral sensitivity matches the most commonly used laser wavelengths. These emulsions are suitable for recording amplitude and phase holograms, and can be used in transmission or reflection mode. A drawback of silver halide emulsions is that they need wet processing and drying.
Dichromated gelatin (DCG) consists of a gelatin layer impregnated with ammonium dichromate. Depending on the development techniques, the hologram can be recorded as a thickness variation or refractive index modulation. Similar to the silver halide emulsions, DCG can be dye sensitized. Among its advantages are large refractive index modulation capability, high spatial resolution, and low scatter. A main disadvantage is the requirement for post recording processing.
Photoresists are light sensitive organic films that yield a surface relief image after exposure and development. Photoresists can also be dye sensitized. Main drawbacks of these holographic recoding materials are the low sensitivity and the post recording processing. Their primary advantage is their low scattering resulting in high spatial frequency resolution.
In addition to these materials for irreversible holographic recording there are a number of materials that may be used several times. Such materials are thermoplastics, photochromic materials undergoing reversible changes in colour when exposed to light, photorefractive materials undergoing change in the refractive index due to light induced charge redistribution resulting in space modulated charge field and photoanisotropic holographic recording materials for polarization holography.
Conventionally, the photosensitive material is prepared as a film or solid layer on a substrate. Since the materials are generally photosensitive, once prepared, they are typically stored in the dark until required for use.
Holograms have been used in a number of different sensing applications. For example, Smart Holograms Ltd. of Cambridge, UK have proposed a number of Holographic sensors for detection of a variety of conditions including moisture, alcohol and analytes. The general principle of detection employs a change in the optical characteristics of already existing hologram. Generally, these sensors employ a preformed hologram formed with a support medium. The support medium is analyte sensitive. In use, a physical property of the hologram support medium is altered by the reaction of the analyte in liquid form with a substance disposed throughout the sensor. This in turn causes a variation of one or more optical characteristics of the hologram, which is detectable. The physical property of the holographic element (principally formed using silver halide based films) which alters may, for example, be its volume, shape, density, viscosity, strength, hardness, charge, hydrophobicity, solvent swellability or integrity. The detectable variation arising from the alteration of the physical property is suitably a change in optical characteristics, including for example, polarisability, reflectance, refractance or absorbance of the holographic element. Examples of patent applications related to this include WO95/26499, WO2003/087899, WO2004/081546, WO99/63408, WO2006/008531 and WO2006/008524.